After completion of the analysis of the human DNA sequence, currently the structural analysis of proteins synthesized from this genetic information as well as post-translationally modified molecules from these proteins has become increasingly important. As a method of the structural analysis, i.e. amino-acids sequence analysis, mass spectrometers are available. Particularly, mass spectrometers composed of ion traps and Q mass filters using a radio frequency (RF) electric field and time-of-flight (TOF) mass spectrometers are high speed analysis tools, and therefore, these have good compatibility with a preseparation device of sample, such as liquid chromatography apparatus. Accordingly, these are suitable for proteomics in which a large number of samples must be continuously analyzed.
In mass spectrometers, sample molecules are ionized and then injected into vacuum (or ionized in vacuum), and mass to charge ratios of target molecular ions are determined by movements of the ions in an electromagnetic field. Since the obtained information represents macroscopic quantities of mass to charge ratios, it is difficult to obtain information on internal structure, or sequence, by a single mass analysis. Accordingly, a method called tandem mass spectrometry is used. That is, sample ions are specified or selected in a first mass analysis. These ions are referred to as parent ions. Subsequently, the parent ions are dissociated by a certain technique. The dissociated ions are referred to as fragment ions. The dissociated ions are further mass analyzed, thereby obtaining some information on generation patterns of the fragment ions. Since there is a rule for dissociation patterns depending on each dissociation technique, it becomes possible to presume the sequence structure of the parent ions. Particularly, in the analysis of biomolecules composed of amino acids, collision induced dissociation (CID), infra red multi photon dissociation (IRMPD), and electron capture dissociation (ECD) are used for the dissociation technique.
CID is currently the most widely used in the protein analysis. Kinetic energy is provided to the parent ions to allow them to collide with gas. Molecular vibrational states are excited by the collision and the molecular chain is dissociated at sites susceptible to cleavage. Further, a method that has recently come to be used is IRMPD. The parent ions are irradiated by infra red laser to allow them to absorb multiple photons. The molecular vibrations are excited and a molecular chain is dissociated at a site susceptible to cleavage. The sites susceptible to cleavage by CID or IRMPD are sites designated as b-y in the backbone consisting of amino acid sequence. It is known that a complete structural analysis can not be carried out only by CID or IRMPD, since even when sites correspond to b-y, those are sometimes hard to be cleaved depending on the kind of amino acid sequence pattern. Therefore, a pretreatment using an enzyme or the like becomes necessary, which hampers high speed analysis. Further, when CID or IRMPD is used for biomolecules with post-translational modification, side chains involved in post-translational modification tend to be easily lost. Due to facile cleavage of the side chains, it is possible to judge molecular species involved in the modification based on lost mass and whether modified or not. However, important information on modification sites concerning which amino acids are modified is lost.
On the other hand, an alternative dissociation technique, ECD, is less dependent on amino acid sequence (as an exception, proline residue with a cyclic structure is not cleaved) and cleaves only one c-z site on the backbone of amino acid sequence. Therefore, a complete analysis of protein sequence can be performed only by mass analysis. In addition, ECD is suitable for research and analysis of post-translational modification owing to its property of hardly cleaving side chains. Therefore, this dissociation technique, ECD, attracts particular attention in recent years.
Electron energy for ECD is known to be approximately 1 eV (Non-patent Document 1). Further, an electron capture reaction is known to occur also near 10 eV. This reaction is referred to as hot ECD (HECD). The reaction that selectively cleaves the c-z site is the former ECD and the latter HECD generates a number of fragment ions cleaved at the c-z site as well as at other sites including the a-x site and b-y site. For this reason, ECD is preferred as a simple analysis technique. However, a combined use of HECD is also studied in a practical analysis. In other words, control of electron energy with accuracy below 1 eV is required to properly use ECD and HECD. As described above, CID and IRMPD and also ECD can be utilized in a mutually complementary manner to provide different sequence information.
Although ECD has been conventionally implemented only by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, a method in which ECD can be implemented in an RF ion trap has started to be reported. The advantage of utilizing the RF ion trap is its performance proven by wide industrial application based on the fact that its device is low in cost and its operation is simple compared with FT-ICR. Here, a conventional technique capable of ECD by FT-ICR, a conventional technique performed in an RF ion trap, and other techniques disclosed in patents are explained.
FIG. 19 is a schematic diagram to explain an example of a basic device structure of ECD by FT-ICR. It includes an ion introduction unit (1909 to 1911) and an FT-ICR unit (1901 to 1908). The ion introduction unit includes linear quadrupole electrodes (represented by the reference numeral 1909) and wall electrodes (1910 and 1911), an RF voltage is applied to the linear quadrupole electrodes, and a positive static voltage with respect to the linear quadrupole electrodes is applied to the wall electrodes, thereby capturing positive sample ions injected (the injection is indicated by an arrow 1912). Only ion species wanted to be measured is isolated from the sample ions in this ion introduction unit. The isolated ions are ejected from the ion introduction unit as shown by an arrow 1913 by applying a voltage lower than that of the linear quadrupole electrodes to the wall electrode 1910 and injected into the FT-ICR unit.
The FT-ICR unit includes a strong magnetic field (typically not weaker than 1 T; lines of magnetic force are indicated by arrows represented by 1908), four pick-up electrodes (1901 to 1904), and two pieces of wall electrodes (1905 and 1906). The isolated ions are captured by the magnetic field in the direction perpendicular to the magnetic field. Further, it is captured by a static voltage applied between the pick-up electrodes and the wall electrodes in the direction parallel to the magnetic field. Electrons generated by an electron source 1907 are injected into an FT-ICR cell and an ECD reaction is induced. Dissociated ions produced by the ECD reaction are measured for their masses by detecting electric currents induced in the pick-up electrodes by cyclotron frequency of the ions.
As described above, FT-ICR does not use a variable electromagnetic field such as RF but uses a static electromagnetic field in order to capture ions. Accordingly, electrons are not accelerated by the electromagnetic field. The use of the static electromagnetic field allows electrons to be led to the trapped ions at a low kinetic energy of 1 eV in a state that the ions are trapped. However, since FT-ICR requires a strong parallel magnetic field (higher than several teslas) with the use of a superconducting magnet, it is high in cost and large in size. Further, in order to obtain one spectrum, the measurement requires several seconds to ten seconds, and the time for the Fourier analysis necessary to obtain the spectrum is approximately ten seconds. It can not be said that FT-ICR that requires several tens of seconds in total has excellent compatibility with liquid chromatography in which one peak appears approximately in ten seconds. In other words, FT-ICR has a disadvantage or difficulty for use in high speed protein analysis. For this reason, the development of ECD technique that does not employ FT-ICR has been awaited.
As one technique for realization of ECD that does not employ FT-ICR, an idea in which an ECD reaction is allowed to occur by passing ions through electron cloud trapped in a Penning trap by a static electromagnetic field is disclosed (Patent Document 1). However, realization of ECD by this technique has not been reported to date.
As another technique for realization of ECD that does not employ FT-ICR, there is an idea in which ions are trapped in an RF ion trap or an RF ion guide and electrons are irradiated thereto. There are patent disclosures related to the idea in which electrons are irradiated to ions trapped in a three-dimensional RF ion trap (Patent Documents 2, 3, and 4). Prior to these disclosures, Vachet et al. tried to realize the reaction of electrons with ions by injecting an ion beam into a three dimensional RF ion trap (Non-patent Document 2); however, the incident electrons were heated by an RF electric field and lost to the outside of the ion trap, thus not giving rise to realization of ECD.
To avoid the problem of heating of electrons in an RF ion trap and an RF ion guide, an idea in which electron trajectories are restricted with the use of a magnetic field is disclosed. In the inside of an RF electric field, a condition to stably capture both ions and electrons can not be practically obtained. Hence, ideas to restrict movements of electrons in the direction perpendicular to lines of magnetic force with the use of a magnetic field have been devised.
One technique has been disclosed by Zubarev et al. (Patent Document 5), in which electron trajectories are restricted by applying a magnetic field to a three dimensional ion trap or an ion guide not having an ion trap function, thus avoiding heating of electrons. Its conceptual diagram is shown in FIG. 17. This includes a three dimensional ion trap (1701 to 1703), an electron source formed of a filament (1709), an ion source (1710), and an ion detector (1708). In the three dimensional ion trap, cylindrical permanent magnets (1704-1706) are embedded. A magnetic field parallel with the central axis is applied by these permanent magnets. First, ions produced by the ion source are trapped in the three dimensional ion trap. Here, parent ions to be measured are isolated from sample ions using resonance excitation of the ions. Electrons produced by the filament electron source are injected into the ion trap to cause an ECD reaction. Ions produced by the reaction are resonantly ejected and detected. Realization of the above ECD reaction by the three dimensional ion trap has been reported (Non-patent Document 3).
Another technique that has been proposed is that electron trajectories are restricted by applying a magnetic field to a linear ion trap in parallel with the central axis thereof and heating of electrons is avoided. Its conceptual diagram is shown in FIG. 18. An ECD reaction unit includes linear quadrupole electrodes (1801), a wall electrode consisting of permanent magnet (1802), another wall electrode (1803), an RF power source (1804), and an electron source unit (1809). The linear ion trap stores ions by means of a quadrupole electric field formed in the inside of the linear quadrupole electrodes by applying RF to the electrodes and a static electric field generated by applying a static voltage to the wall electrodes. Electrons are injected thereto. At this time, the electrons are injected along the central axis of RF. Since an RF electric field on the central axis is zero, the ions are not influenced by the RF electric field in the vicinity of the central axis or even when influenced, its effect is small. Further, a magnetic field generated by the permanent magnet 1802 is applied in parallel with the central axis. Thus, even when the electrons travel from the central axis, those are captured by the magnetic field, and thus their trajectories do not deviate from the central axis to a significant degree. In this way, heating of electrons are avoided. Since the present disclosure assumes that this ECD reaction unit is inserted between an ion source and another mass analysis unit represented by a TOF mass analysis unit, the electron source (1809) and an ion source (incidence of ions is shown by an arrow 1806) are combined by inserting a quadrupole deflector (1808) at one ion inlet of the linear ion trap. Ions produced by a reaction are ejected from the linear ion trap and then injected into said another mass analysis unit as shown by an arrow 1807 (Non-patent document 4).
[Patent Document 1] U.S. Patent No. 20040245448
[Patent Document 2] U.S. Pat. No. 6,653,622
[Patent Document 3] U.S. Patent No. 20040232324
[Patent Document 4] PCT/DK02/00195
[Patent Document 5] U.S. Pat. No. 6,800,851
[Patent Document 6] JP-A No. 021871/1998
[Patent Document 7] JP No. 03361528
[Non-patent Document 1] Frank Kjeldsen et al. Chem. Phys. Lett. 2002, vol. 1356, p2001-2006
[Non-patent Document 2] R. W. Vachet, S. D. Clark, G. L. Glish: Proceedings of the 43th ASMS conference on Mass Spectrometry and Allied Topics (1995) 1111
[Non-patent Document 3] Zubarev, R. A. et al. JASMS 2005, vol. 16, p22-27
[Non-patent Document 4] Takashi Baba et al. Analytical Chemistry 2004, vol. 76, p4263-4266
[Non-patent Document 5] Proceedings of the ASMS Conference on Mass Spectrometry 2003 (Th PL1 165)
[Non-patent Document 6] J. C. Schwartz et al. J. Am. Soc. Mass Spectom. 2002, vol. 13, p659